EP2374136A2 - Method for processing a nitrous aqueous liquid effluent by calcination and vitrification - Google Patents

Abstract

The invention relates to a method for processing a nitrous aqueous liquid effluent containing nitrates of metals or metalloids, including a step of calcinating the effluent in order to convert the metal or metalloid nitrates into metal or metalloid oxides, at least one compound selected from the metal or metalloid nitrates and the other compounds of the effluent resulting in a sticky oxide upon calcination, and a dilution additive resulting in a non-sticky oxide upon calcination being added to the effluent prior to the calcination step, wherein the dilution additive includes aluminium nitrate and at least one nitrate selected from iron nitrate and rare earth nitrates.

Description

 PROCESS FOR TREATING NITRIC AQUEOUS LIQUID EFFLUENT BY CALCINATION AND VITRIFICATION

DESCRIPTION

The invention relates to a process for the treatment of a liquid aqueous nitrate effluent, generally containing mainly sodium nitrate with nitrates of metals or metalloids, which comprises a calcination step generally followed by a vitrification step of the calcine obtained during said calcination step.

The technical field of the invention can be defined generally as that of the calcination of liquid effluents, more particularly the technical field of the invention can be defined as the calcination of radioactive liquid effluents for their vitrification.

The French vitrification process for radioactive liquid effluents comprises two steps. The first step is a step of calcining the effluent during which a drying and then a denitration of a portion of the nitrates takes place. The second step is a vitrification step by dissolving in a glass of containment the calcine produced during the calcination step.

The calcination step is generally carried out in a rotary tube heated to 400 ^° C. by an electric furnace. Solid calcine is crushed by a crazy bar placed inside the rotating tube.

During the calcination of certain solutions, in particular solutions rich in sodium nitrate, in other words solutions with a high sodium content in a nitric medium, it is possible to observe a bonding of the calcine on the walls of the rotating tube which can lead to a total clogging of the calciner tube. The parade was to add to the effluent a compound deemed non-sticky, aluminum nitrate, to allow their calcination by avoiding the clogging of the calciner.

But this aluminum nitrate added to the effluent increases the amount of glass to be produced. Indeed, the presence of alumina in the glass increases its production temperature and leads to limit the rate of waste load, effluent in the glass, so as not to degrade the confinement properties of this glass. The aluminum content in the glass must not be too high and is generally limited to about 15% by weight expressed as Al 2 O 3.

The amount of aluminum nitrate to be added is also difficult to optimize, so for each new effluent, several tests are necessary to determine the operating conditions of heated rotating tube calcination to avoid clogging of the tube. In particular, it is necessary to adjust the heating of the calcination furnace and the quantities of calcination aid, which is different dilution adjuvant, which is very often sugar.

There is therefore a need in view of the foregoing for a treatment process by calcination of a nitric aqueous effluent containing compounds, such as nitrates of metals or metalloids and other compounds, likely to form sticky oxides during of their calcination, which makes it possible to avoid calcine bonding on the walls of the calcination tube and the clogging of this calcination tube and which simultaneously limits the increase in the quantity of confining glass to be produced during vitrification calcine.

More particularly, there is a need for a process for the treatment of effluents capable of causing a sticking during their calcination, using a dilution adjuvant, which, while avoiding the bonding of the calcine on the walls of the apparatus of calcination and clogging of the latter, at least as efficiently as aluminum nitrate, does not increase the amount of glass to be produced, and does not limit the rate of charge of the glass in waste.

In particular, there is a need for a process for the treatment of effluents containing compounds, such as nitrates of metals or metalloids and other compounds, generating sticky oxides during their calcination, in particular solutions with a high nitrate content. sodium, which avoids clogging of the calcination tube and reduces the constraints imposed on the glass formulation, these constraints being due to the addition of aluminum in the form of aluminum nitrate in the calcination aid.

The object of the present invention is to provide a method for treating an aqueous nitrate liquid effluent containing nitrates of metals or metalloids, this process comprising a step of calcining the effluent to transform the nitrates of metals or metalloids into their oxides, which, among other things, meet the needs mentioned above.

The object of the present invention is still to provide such a method which does not have the drawbacks, limitations, defects and disadvantages of the processes of the prior art and which solves the problems of the processes of the prior art, in particular methods aluminum nitrate is used as a dilution adjuvant.

This and other objects are achieved in accordance with the invention by a method of treating an aqueous nitrate liquid effluent containing nitrates of metals or metalloids, comprising a step of calcining the effluent to transform the nitrates of metals or metalloids to oxides of metals or metalloids, at least one compound selected from nitrates of metals or metalloids and other compounds of the effluent resulting in calcination with a sticky oxide, and a dilution adjuvant leading during calcination with a non-sticky oxide being added to the effluent prior to the calcination step, in which process the adjuvant dilution comprises aluminum nitrate and at least one other nitrate selected from iron nitrate and rare earth nitrates.

Advantageously, the dilution adjuvant consists of aluminum nitrate and at least one other nitrate selected from iron nitrate and rare earth nitrates.

The process according to the invention is fundamentally characterized by the use, during calcination, of a particular dilution adjuvant which comprises, in addition to aluminum nitrate, at least one specific nitrate chosen from iron nitrate and rare earth nitrates.

The use of iron nitrate or a rare earth nitrate in a dilution adjuvant added to a liquid aqueous nitrate effluent prior to the calcination of this effluent has hitherto never been mentioned or mentioned.

It has been found that, surprisingly, iron nitrate and rare earth nitrates have calcination bond limiting properties close to those of aluminum nitrate, but that the oxides derived from these specific nitrates, which are so-called "non-sticky" oxides, can also dissolve in the final glass produced during the subsequent vitrification step.

The use of a dilution adjuvant comprising a nitrate chosen from iron nitrate and rare earth nitrates in substitution for part of the aluminum nitrate thus makes it possible to prevent clogging of the calcination apparatus tube. during the calcination of effluents generating highly sticky oxides, such as solutions with high sodium content, while minimizing the increase in the amount of confining glass to be produced during the vitrification step which generally follows the calcination.

Surprisingly, iron nitrate and rare earth nitrates exhibit all the excellent properties of aluminum nitrate in its ability to limit calcine bonding, and thus to avoid clogging of the calcination tube. and have an advantage in reducing the amount of glass to be produced and increasing the rate of waste charge incorporated in the glass.

The constraints imposed on the glass formulation by the dilution adjuvants according to the invention comprising, in substitution of a portion of the aluminum nitrate, at least one specific nitrate selected from iron nitrate and rare earth nitrates are markedly reduced. compared to dilution adjuvants consisting solely of aluminum nitrate because of the lower or no aluminum intake. Rare earth nitrates are generally chosen from lanthanum nitrate, cerium nitrate, praseodymium nitrate, and neodymium nitrate; and therefore the dilution adjuvant may advantageously comprise aluminum nitrate and at least one other nitrate selected from nitrate of iron, lanthanum nitrate, cerium nitrate, praseodymium nitrate and neodymium nitrate.

Advantageously, the dilution adjuvant consists of aluminum nitrate and at least one other nitrate selected from iron nitrate, lanthanum nitrate, cerium nitrate, praseodymium nitrate and neodymium nitrate.

A particularly preferred dilution adjuvant according to the invention consists of aluminum nitrate and iron nitrate.

Another particularly preferred dilution adjuvant according to the invention consists of aluminum nitrate, lanthanum nitrate, neodymium nitrate, cerium nitrate and praseodymium nitrate.

The respective amounts of each of the aluminum, iron, and rare earth nitrates are free from the point of view of their effectiveness in preventing the bonding of the calcine in the tube and can therefore be adjusted according to their impact on the properties of the containment glass prepared in a subsequent vitrification step.

The amount of dilution adjuvant added to the liquid effluent is a function of the contents of sticky compounds of the liquid effluent (nitrates and / or other compounds), expressed in terms of oxides, of the total mass of the nitrates (or possibly more precisely, the total mass of salts), also expressed in terms of oxides, contained in the effluent. The effluent is generally composed mainly of a mixture of metal nitrates and metalloids with a majority of sodium nitrate and may also contain a quantity of aluminum nitrates, iron and rare earths in contents insufficient to prevent clogging of the tube during the calcination step.

The effluent may also contain "sticky" or "non-sticky" compounds which are not nitrates, generally present in the form of salts, such as phosphomolybdic acid which is a so-called "sticky" compound.

The method according to the invention because of the implementation of the specific dilution adjuvant mentioned above allows the calcination without clogging of all kinds of effluents, whatever their nature and the nature of the nitrates and nitrates sticky which there are contained.

The liquid effluent treated by the process according to the invention contains at least one compound such as a nitrate of metal or metalloid leading during calcination to a so-called "sticky" oxide, such as sodium nitrate, and / or another compound (which is not a nitrate) leading during calcination to a so-called "sticky" oxide.

In the present description, the terms "sticky compounds", "sticky oxides" or "sticky nitrates" are used.

The term "sticky compounds", "sticky nitrates" or "sticky oxides" means compounds, oxides, nitrates known to stick to the walls of "calciner" calcination apparatus and induce clogging phenomena of these calcinators.

The terms "sticky compound", "sticky oxide", "sticky nitrate" are terms commonly used in this field of the art, which have a well-established meaning, which are known to those skilled in the art and do not present for him any ambiguity.

Thus, the compound (s), such as (the) nitrate (s) and / or the other compound (s), which leads (conduct) upon calcination to may sticky oxide (s) be (may they) be selected from sodium nitrate, phosphomolybdic acid, boron nitrate and mixtures thereof.

The content of the compound (s), such that (these) nitrate (s) and other compound (s) leading during the calcination to (or) "sticky" oxide (s) in the effluent, expressed as oxide, relative to the total mass of salts, including nitrates, contained in the effluent, also expressed as oxide, is generally greater than 35% by mass.

The process according to the invention makes it possible, in particular, for the calcination of effluents having a high content of nitrates and other compounds, called "stickiness", ie greater than 35% by weight, expressed as oxides.

Particularly advantageously, the process according to the invention allows the calcination of high sodium solutions which are very tacky. "High sodium", more specifically sodium nitrate, is generally understood to mean that the effluent has a sodium nitrate content, expressed as sodium oxide Na2 <0, relative to the total mass of the salts, including the nitrates contained in the effluent, expressed in oxides, greater than 30% by weight, preferably greater than 50% by weight.

The conditions of the calcination step are known to those skilled in the art and can easily be adapted depending on the nature of the effluents treated.

The conditions of this calcination, apart from the notable fact that any clogging is avoided, are not fundamentally modified by the implementation of the specific calcination adjuvant according to

The invention.

The calcination conditions are generally as follows: temperature reached by the calcine at about 400 ^° C., rotation speed of the tube at 40 rpm, addition of a calcination aid, for example of the sugar type.

This calcination step is generally carried out in a heated rotating tube, for example a rotating tube heated by an electric furnace with several independent heating zones. Heating zones are more particularly dedicated to evaporation and others to calcination.

Calcining zones can heat the calcine at a temperature of about 400 ⁰ C. The speed of rotation of the tube, the addition of the calcination aid and the presence of an idle bar makes it possible to divide the solid calcine so that it can react under good conditions in the vitrification unit.

The treatment process according to the invention generally comprises, after the calcination step, a vitrification step of the calcine obtained during this calcination step. This vitrification step consists of a reaction between the calcine and a glass frit (preformed glass) to obtain a confinement glass.

In other words, after the calcination step, a vitrification step is carried out which consists of producing a confinement glass from the melting of the calcine produced during the calcination step with glass frit.

As already mentioned above, the use in the dilution adjuvant of specific nitrates of iron and rare earths makes it possible to relax the constraints with regard to the formulation of the glass. In particular, a higher proportion of effluent in the glass can be incorporated when the calcine was obtained using the dilution adjuvant according to the invention in place of a dilution adjuvant consisting solely of aluminum nitrate.

In other words, the binding limit on the rate of incorporation of effluents into the glass, due to aluminum nitrate, is eliminated, and the rate of incorporation is significantly increased and passes through. example of 13% by weight of oxides at 18% by weight of oxides relative to the total mass of the glass.

In addition, the high intake of aluminum in the case of a dilution admixture consisting solely of aluminum nitrate tends to harden the calcine and has the consequence of causing a decrease in the reactivity between the calcine and the frit of glass in the vitrification furnace.

On the contrary, the addition of iron makes the calcine more friable and therefore easier to vitrify.

Vitrification consists of a melting reaction between the calcine and the glass frit to form a confining glass.

It is carried out in two types of furnaces: indirect induction furnaces which consist in heating by four inductors a metal pot into which the sintered / calcined mixture is introduced, and the direct induction furnaces which consist of heating the glass by an inductor to through a cooled structure (cold crucible) which passes a part of the electromagnetic field and in which is introduced continuously the sintered mixture / calcinate.

The invention will now be described with reference to the following examples, given for illustrative and not limiting.

Example 1 (comparative):

In this example, the calcination of an effluent containing a high content of sodium nitrate is described. The composition of this effluent (waste) is given in Table 1, this composition being expressed in mass% of the oxides corresponding to the salts contained in the effluent, which are mainly nitrates.

The percentage of oxides is expressed in relation to the total mass of the oxides corresponding to the salts contained in the effluent.

The effluent described in Table 1 below is very charged especially in sodium and therefore very sticky.

To this effluent is added an adjuvant (adjuvant 1) of the prior art which consists of 100% by weight of aluminum nitrate expressed as Al ₂ O ₃ oxide.

The conditions of calcination are as follows:

Calcinator with four independent heating zones, the temperature reached by the calcine is about 400 ^° C., the speed of rotation of the rotating tube containing the idler bar is 20 rpm, the calcination aid content is 40 g Of the mixture of the effluent with the dilution adjuvant.

Example 2

In this example, the calcination of the same effluent as that of Example 1 and described in Table 1 is carried out.

To this effluent is added an adjuvant (adjuvant 2) according to the invention which consists of 75% by weight of aluminum nitrate expressed as Al 2 O 3 oxide. and 25% by weight of iron nitrate expressed as Fe ₂ O ₃ oxide.

The conditions of the calcination are the same as that of Example 1.

TABLE 1

Example 3 (comparative):

In this example, the vitrification of the calcine obtained in Comparative Example 1 is carried out.

Recall that this calcine was prepared using an adjuvant ("adjuvant No. 1") consisting solely of aluminum nitrate.

The production of a glass from calcine and a glass frit containing 1% by weight of alumina, the proportion of frit in the glass being 77.43%, leads to a maximum incorporation rate of the initial waste into the glass of 11.6% given by the following calculation ((100-51, 27) ) * (13-1) / (51, 27-1)).

Example 4

In this example, the vitrification of the calcine obtained in Example 2 according to the invention is carried out. Recall that this calcine was prepared using an adjuvant ("adjuvant No. 2") consisting of 75% by weight of aluminum salt and 25% by weight of iron salt.

It was determined that the maximum incorporation rate of the initial waste (thus before mixing) is limited to 11.6% of the mass of the glass in Comparative Example 3, whereas in Example 4, the rate of maximum incorporation is 15.6%.

In addition, the high intake of aluminum by the adjuvant No. 1 tends to harden the calcine and results in a slight decrease in reactivity between the calcine and the glass frit in the vitrification furnace.

On the contrary, the addition of iron with adjuvant No. 2, according to the invention, makes the calcine more friable and therefore easier to vitrify.

Example 5

In this example, the calcination of an effluent constituted by 100% of sodium nitrate described in Table 2 is described. In a first experiment, there is added to this effluent a builder (adjuvant 1) of the prior art which consists of 100% by weight of aluminum nitrate expressed as Al 2 O 3 oxide. In a second experiment, the calcination of sodium nitrate was carried out with an adjuvant

(Adjuvant 3) according to the invention in which part of the aluminum nitrate has been replaced by a mixture of nitrates of lanthanum, cerium, neodymium and praseodymium.

In both cases, the content of sodium nitrate expressed as total oxide mass represents 30% in the mixture of the effluent with the dilution adjuvant. The conditions of calcination are as follows:

Calcinator with two independent heating zones, the temperature reached by the calcine is approximately 350 ^° C., the speed of rotation of the rotating tube containing the idler bar is 35 rpm, the calcination aid content is 20 g Of the mixture of the effluent with the dilution adjuvant.

TABLE 2

Claims

A method of treating an aqueous nitrate liquid effluent containing nitrates of metals or metalloids, comprising a step of calcining the effluent to transform the nitrates of metals or metalloids into oxides of metals or metalloids, at least one compound selected from the nitrates of metals or metalloids and the other compounds of the effluent resulting in the calcination with a sticky oxide, and a dilution adjuvant leading, during calcination, to a nonstick oxide being added to the effluent prior to the calcination step, wherein the dilution adjuvant comprises aluminum nitrate and at least one other nitrate selected from iron nitrate and rare earth nitrates. The process according to claim 1, wherein the dilution adjuvant comprises aluminum nitrate and at least one other nitrate selected from iron nitrate, lanthanum nitrate, cerium nitrate, praseodymium nitrate, and neodymium nitrate. 3. Method according to any one of the preceding claims wherein the at least one compound leading on calcination to a tacky oxide (s) is (are) chosen from sodium nitrate, phosphomolybdic acid, boron nitrate, and mixtures thereof. 4. Method according to any one of the preceding claims, wherein the content of compound (s) leading on calcination to a (or) oxide (s) sticky (s), expressed in oxide, relative to the total mass salts contained in the effluent, expressed as oxide, is greater than 35% by weight. 5. Process according to claim 3, in which the effluent has a sodium nitrate content, expressed as sodium oxide Na2 <0, relative to the total mass of the salts contained in the effluent, expressed in oxide, greater than 30% by weight, preferably greater than 50% by weight. 6. Method according to any one of the preceding claims, wherein the calcination step is carried out in a heated rotating tube, allowing the calcine to reach a temperature of about 400 ^° C. 7. Method according to any one of the preceding claims, wherein after the calcination step is carried out a vitrification step which consists in developing a confinement glass from the melting of the calcine produced during the calcination step with glass frit.